The present invention generally relates to optical bistable laser diodes and more particularly to the structure of such a laser diode and a control method for operating such diode at a high speed.
Recently, optical bistable laser diodes have received attention as essential elements for constructing optical logic devices or optical memory devices for use in optical information processing systems or optical computers. Such devices having optical bistability are suited for optical information processing based on binary logic as the state of the device is switched between an optical high level state and an optical low level state similarly to the flip-flops used in conventional electronic processing systems.
FIG. 1 shows a typical prior art bistable laser diode. Referring to the drawing, the device comprises a clad layer 11 of a semiconductor material doped to a first conduction type, an active layer 12 of undoped semiconductor material provided on the clad layer 11, and another clad layer 13 doped to another conduction type further provided on the active layer 12. On the clad layer 13, there are provided first and second electrodes 14a and 14b forming a tandem electrode structure respectively in correspondence to a gain region 15 and a control region 17, both defined in the active layer 12. The gain region 15 is supplied with a drive current I.sub.1 through the first electrode 14a and produces optical radiation by photon emission. The control region 17 is also supplied with a control current I.sub.2 through the second electrode 14b and controls the overall gain of the laser diode. In the prior art device, the level of the control current I.sub.2 is set such that the control region 17 operates as a second gain region. In other words, the control region 17, also, produces optical radiation with a controlled gain by emitting photons. Further, there is defined a saturable absorption region 16 between the gain region 15 and the control region 17. This region 16 changes its transmittance responsive to the optical radiation in the active layer 12 such that the transmittance of the layer 12 is increased responsive to an increase of the optical radiation in the active layer 12 and that the transmittance is decreased responsive to a decrease of the optical radiation in the active layer 12. Furthermore, there is provided a rear side electrode 18 at the bottom of the clad layer 11 for collecting electrical current injected through the first and second electrodes 14a and 14b. Note that a pair of mirrors E.sub.1 and E.sub.2 are provided at respective ends of the laser diode to form a Fabry-Perot resonator as usual.
In a typical example, the laser diode has an overall length of 300 .mu.m across the mirrors and the gain region 15 has a length of 192 .mu.m, the saturable absorption region 16 has a length of 38 .mu.m and the control region 17 has a length of 70 .mu.m.
Next, the operation of this prior art optical bistable laser diode and its problems will be described.
In operation, the gain region 15 and the control region 17 are first supplied with the drive current I.sub.1 and the control current I.sub.2 respectively as already described such that there is photon emission in both of the regions 15 and 17. When the overall gain of the regions 15 and 17 exceeds the loss in the saturable absorption region 16 as well as the loss at the mirrors E.sub.1 and E.sub.2, the laser starts to oscillate, and when the loss exceeds the gain in the regions 15 and 17, the oscillation of the laser stops. More specifically, in a first optical bistable state corresponding to the optical low level state of the laser diode, the optical radiation established in the active layer 12 as a result of the photon emission in the regions 15 and 17 is insufficient to cause transition of the saturable absorption region from opaque to transparent and no laser oscillation occurs. As a result, only a small optical output due to the electroluminescence is obtained. In the second bistable optical state corresponding to the optical high level state, the saturable absorption region becomes substantially transparent as a result of the strong optical radiation established in the active layer 12 and a large optical output power associated with the decrease of the loss in the saturable absorption region is obtained.
FIG. 2 is a characteristic curve of the laser diode of FIG. 1 showing the optical output as a function of the overall current I injected to the laser diode. This current I is the sum of the drive current I.sub.1 and the control current I.sub.2 and is first set at a current level I.sub.B as shown in the drawing. The laser diode is turned on by increasing the overall current I beyond a turn-on level I.sub.ON and is turned off by decreasing the overall current I below a turn-off level I.sub.OFF. Such a turn-on and turn-off of the laser diode can be achieved by increasing the control current I.sub.2 so that the overall current I exceeds the turn-on level I.sub.ON and by decreasing the control current I.sub.2 so that the overall current I falls below the turnoff level I.sub.OFF. In other words, the laser diode is turned on by supplying a set current pulse having a level I.sub.ON -I.sub.B or more to the control region 17 in addition to the foregoing current I.sub.2 and is turned off by supplying a reset current pulse having a level I.sub.B -I.sub.OFF to the control region 17. As the level I.sub.ON and I.sub.OFF are not identical, there appears a hysteresis which characterizes the bistable operation of the laser diode. Thus, in the turned-on state, the optical output power of the laser diode assumes a level Q corresponding to the optical high level state, while in the turned-off state, the optical output power assumes a level P corresponding to the optical low level state.
Alternatively, the laser diode of FIG. 1 may be turned on by injecting an external optical beam P.sub.in having a wavelength chosen so as to cause interaction with the material forming the active layer 12. More specifically, the optical beam P.sub.in has a wavelength or an energy which is equal to or larger than the band gap in the active layer 12. In this case, the current I.sub.1 and I.sub.2 are each set such that the overall current I assumes the level I.sub.B similarly to the foregoing case and the laser diode is triggered by injecting an optical beam. When an optical beam called a set optical pulse is injected, the saturable absorption region 16 absorbs the optical beam and generates carriers. The carriers thus generated are accumulated in the region 16. As a result, the saturable absorption region 16 reduces its absorption coefficient and the region 16 changes from opaque to transparent. Thus, strong laser oscillation is established along with an optical output corresponding to the optical high level state. Further, by applying the reset current pulse already described to the control region 17, the carriers are removed from the region 17 and the oscillation of the laser diode stops because of the loss in the control region 17 which is now depleted of carriers. Note that the saturable absorption region 16 recovers its region transmittance or opacity responsive to a decrease in the optical power in the active layer 12.
In such a prior art optical bistable laser diode, it was necessary to remove the carriers from the control region 17 as quickly as possible in order to obtain a quick turn-off. For this purpose, a very large negative reset current pulse has been used. For example, the current I.sub.2 injected to the control region 17 is almost shut down in correspondence to the reset pulse. However, such a large reset pulse causes a strong depletion of carriers in the control region 17 and an excessively large optical power is needed for the set optical pulse to turn on the laser diode, particularly when the laser diode is to be turned on immediately after turn-off. Otherwise, one has to wait for a long time until the carrier density in the active layer 12 returns to a stationary state. This means that there is a dead time responsive to the reset current pulse as illustrated in FIG. 3 as long as an ordinary or small optical power is used for the optical set pulse. Generally, this dead time has a duration in the order of about several nanoseconds and prohibits the high speed operation of the device.
In optical computers and other information processing systems, it is necessary to operate the optical bistable device with a speed as high as possible without using excessive optical power for the set optical pulse, as the use of a large optical power increases the size and power consumption of the system. From this view point, the prior art optical bistable laser diodes are inappropriate for optical computers. An optical bistable device which can be turned on and turned off at a high speed without using large optical power for the set optical beam is strongly demanded.